How did dinosaurs look? The only way any of us know is from looking at images created by paleoartists, people who specialize in imagining extinct creatures by studying their skeletons. The problem is that skeletons only tell us part of the story, revealing little about layers of body fat, skin type, coloration, and behavior. Now, a new book called All Yesterdays: Unique and Speculative Views of Dinosaurs and Other Prehistoric Animals — half science, half-science fiction — offers us a radical new way of looking at dinosaurs, based on contemporary scientific speculation. We have an incredible gallery of the paleoart from the book.

Written by paleoartists C.M. Kosemen and John Conway, with an introduction by renowned paleontologist Darren Naish, All Yesterdays is the kind of wonderful, provocative thought experiment that only exists at the nexus of science and art. The book corrects a lot of misconceptions from famous dinosaur art, such as work by Charles Knight, and then heads off into new speculations based on all the "unknown unknowns" of paleontological reconstruction. As Naish writes in the book's introduction:

Charles R. Knight was a wildlife illustrator whose career spanned the era when dinosaurs first…
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It should be noted that there are some disagreements at the level of reconstructing skeletons and musculature, and that improvements and tweaks are frequently being made. We mostly agree on the positions of muscles, for example, but the sizes of some of the muscles involved are variable in living animals and there is sometimes no reliable way of determining their size in fossil animals . . .

We have little to go on, but what we know suggests that, yes, integumentary coverings may have effectively obscured much of the underlying anatomy that we've worked so hard to reconstruct. Notably, dinosaurs found with soft tissues (namely skin impressions and feathers) are flamboyant. Feathered dinosaurs are not only covered in feathers (with feathering extending from the middle of the snout all the way to the tip of the tail and even down to the ankles or toes), they have especially long, showy feathers growing off their arms and hands, the end parts of their tails, and even (in cases) from their thighs, shins and feet. Fossil mammals with body outlines and fur show a thick halo of tissues surrounding the skeleton, meaning that the skeleton was deeply submerged and effectively invisible in the live animal, as is typically the case in modern species. We are therefore presented with a huge diversity of ‘known unknowns' and ‘unknown unknowns' – the gate is open for all manner of bizarre possibilities as goes the life appearances of fossil animals.

It is these speculative possibilities that John Conway and C. M. Kosemen have explored in this book.

Palaeontologists and palaeoartists talk about these sorts of ideas all the time-about the possibility that extinct animals were insanely flamboyant, that they had super-sized genitalia, or that they were insulated from the cool or even cold environments they sometimes inhabited by fat, thick skin, or fuzzy coats-but this is the first time ideas of this sort have been extensively discussed in print.

This is a Majungasaurus crenatissimus, by C.M. Kosemen. One of the points that Koseman and Conway make in the book is that there are certain stereotypical images we get of dinosaurs. We always see them in profile, because that's how skeletons look best. Here we see this fierce predator from the front, and get a good view of its crazy wattles.

Stegosaurus and friend, by C.M. Kosemen. Another thing you NEVER see in paleoart is what exactly dinosaurs might have looked like when they were mating. This picture has the virtue of being cute as well as suggestive. The first image you saw in this post, of the T. rex sleeping, is another thing you basically never see in paleoart. We always witness T. rex chomping on its prey. And yet predators spend most of their days sleeping and digesting, preparing for the next high-energy hunt. So your typical T. rex pose would probably have been more like that adorable cat curl than a bloody fight.

This is a Leaellynasaura amicagraphica, by John Conway. This is an experiment with imagining an alternative to what these small dinosaurs might have looked like. The artists note that paleoartists always assume that dinosaurs were sleek, their bodies in the exact shape of their skeletons. But few animals today are shaped exactly like their skeletons. Maybe some dinos were fat with giant, tufted tails.

This is a plesiosaur, by John Conway. Here the artist is imagining this giant ocean predator in an atypical paleoart pose — it's hiding in the muck and algae near the shore, waiting for prey. It's very likely dinosaurs would have had camouflage, and therefore when we imagine them we should consider that their coloration might have matched the colors in their habitats.

These are proceratops, by John Conway. These are a small species related to Triceratops, and we know very little about how they acted. Kosemen and Conway point out that we should look to contemporary animal behavior for clues. They note that goats climb trees, even though their skeletons don't suggest tree climbing animals. Maybe proceratops was the same way?

This is a picture of the infamous Homo diluvii, by C.M. Kosemen. He and Conway share this as an example of how easy it is to "read into" a skeleton whatever you want. The Homo diluvii was something that 18th century scientists first sketched, as a way to explain the skeleton of a giant salamander they'd discovered (the fossil has since been properly identified). Because they couldn't believe a salamander could have been that big, they drew this humanoid and decided it was a radically new kind of human who had lived on Earth long ago. Kosemen and Conway hint that our current paleoart of dinosaurs might be just as laughable and mistaken as this Homo diluvii.

In the final section of All Yesterdays, we go into the realm of science fiction. An imaginary dinosaur paleontologist discovers ancient fossils from the late Quaternary and tries to sketch what they might have looked like. Here, Kosemen shows the way this dino paleontologist might reconstruct a baboon, by assuming that its body was in the exact shape of its skeleton and that it probably looked kind of like a reptile.

Here is John Conway's representation of how our dinosaur paleontologist would draw a cow, based on its skeleton. Obviously it must have been a sleek, muscular animal!

And here is a house cat, drawn by Conway channeling the dinosaur paleontologist. No creature could possibly have fur on its face, since dinosaurs don't. So this is clearly how a cat looked.